The present invention relates to a non-reusable hypodermic syringe. As hypodermic syringes are used for inoculating against infectious diseases and administering prescribed medicines, the advantages of a non-reusable syringe are obvious. With the widespread use of illicit drugs and the spread of Acquired Immune Deficiency Syndrome, commonly know as AIDS, the present need for such a device is acute. While efforts have heretofore been made to provide such a device, as seen in U.S. Pat. Nos. 4,781,684, 4,832,693 and 4,850,968, such efforts have not met with widespread success as a result of deficiencies in the designs of the mechanisms developed to render syringes non-reusable.
A non-reusable syringe should not only be tamper-proof such that the means by which the device is rendered non-reusable can not be removed, but should also self-destruct without any cooperation of the user. For example, the syringe should not require a completion of the full stroke of the plunger within the barrel of the syringe to render the device inoperative as in the devices disclosed in the first two above-cited patents, or one could then leave the barrel of the syringe half full and continue to re-fill the upper part of the barrel. Second, it is important that syringe be capable of acting as an air pump without rendering the syringe inoperative in order to maintain the interior of the vials of medicine at atmospheric pressure. When medicine is withdrawn from a standard vial of medicine by a hypodermic syringe, a negative pressure is created within the vial due to the air-tight seals used on such vials. Repeated withdrawal of medicine from the vial will build up a significant negative pressure within the vial. As the hypodermic needle on a syringe is withdrawn from a vial in which such a negative pressure has been created, air is aspirated into the syringe through the needle, creating small bubbles of air in the liquid medicine with the syringe which is extremely hazardous with intravenous injections. In addition, many injections require a combination of two medicines drawn from separate vials. If a negative pressure is allowed to build up within the vials, contamination and mixing problems will result as well as air bubbles. After drawing into the syringe the desired volume of medicine from the first vial and then inserting the syringe needle into a second vial which has a negative pressure therein, medicine will be drawn from the syringe back through the needle into the second vial, contaminating the medicine in the second vial and making an accurate measuring of the proportions of the two medicines in the syringe extremely difficult. To avoid these problems, it is common practice with conventional hypodermic syringes to first draw into the syringe a volume of air equal to the volume of medicine to be extracted from a particular vial and then inject that measured volume of air into the vial. Upon subsequently withdrawing an equal volume of medicine from that vial, the pressure within the vial will be left at atmospheric. In the single use hypodermic syringes heretofore developed, the means which render such devices non-reusable will act to render the syringe inoperative if one were to attempt to use it as an air pump in an effort to maintain the interior of the vial at atmospheric pressure. As a result, such devices are not only highly susceptible to the ingestion of air, but are also illsuited for injections requiring a mixture of medicines.
The non-reusable hypodermic syringe disclosed herein provides a single solution to all of the above problems by differentiating between the density of fluids therein and destructing solely in response to the ejection of liquid therefrom. The fluid density responsive destruct mechanism of the present invention will not activate upon the passage of air into and out of the syringe so that the syringe freely used as an air pump prior to any liquid being drawn therein to prevent negative pressure from building up in the vials of medicine. However, once a liquid has been drawn into the syringe and even a portion thereof ejected, the destruct mechanism is activated and no additional fluid, air or liquid, can be drawn into the syringe, rendering the device inoperable other than to eject the remaining liquid within the syringe.